fix: harden frailbox logger error fallback handling - #29

Closes #20.

Summary

Improves the legacy frailbox logger fallback path so file-open and file-write failures preserve log output on stderr instead of silently losing messages.

Changes

• Added explicit stderr fallback state for failed LOG_FILE opens.
• Added write/flush failure handling that reports the failing operation, path, errno text, and likely cause before replaying the current log line to stderr.
• Added shutdown flush/close diagnostics for file operation failures.
• Made log_dump_ring_buffer return -1 when its descriptor write fails.
• Added frailbox/tests/test_logger_errors.c covering missing-path open fallback and /dev/full write fallback.
• Added make logger-error-test for the focused harness.
• Documented the logger fallback behavior and harness command in docs/OPERATIONS.md.

Testing

• cd frailbox && make logger-error-test
  • Not completed in this sandbox because gcc and make are not available.
• python3 build.py
  • Generated diagnostic/build-fa564fee.logd.
  • Generated diagnostic/build-fa564fee.json.
  • The build runner reported missing local toolchains in this environment (cargo, npm, go, gcc, g++, cmake, make, javac, ruby, luac, ghc), so module builds did not complete here.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

• I would like to request that my diagnostic build log is removed before merging